understanding how IDP login works

Fri Nov 5 21:04:03 UTC 2021

Hello,
How exactly does the login sessions work? I notice that if I log into sp1, then open a new tab and go to sp2 I am automatically logged in. Then if I open another new tab and go to sp3 I have to log in again. What/where controls if you have to reauthenticate in or not?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://shibboleth.net/pipermail/users/attachments/20211105/238badcd/attachment.htm>